Far-infrared properties of ab-plane oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Polycrystalline samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with a variety of surface treatments show differences in absolute reflectance and width of phonon lines. Samples that are not polished and are measured immediately after annealing have largely grains with the c axis normal to the surface. Such oriented samples show a gaplike depression of conductivity in the far infrared that sets in below the superconducting transition temperature but no true gap. Phonon lines at 195 ${\mathrm{cm}}^{\mathrm{-}1}$ and at 155 ${\mathrm{cm}}^{\mathrm{-}1}$ narrow in the superconducting state, in analogy with the effect of the electron-phonon interaction in Bardeen-Cooper-Schrieffer superconductors. In the normal state the background conductivity is Drudelike with a plasma frequency of 0.75 eV and a relaxation rate of 200 ${\mathrm{cm}}^{\mathrm{-}1}$. The extrapolated far-infrared conductivity agrees with the measured dc conductivity.